1. Field of the Invention
This invention relates to a process for making radial tires, and more particularly to a process for making radial tires having a carcass including one or more rubberized cord plies whose cords are disposed substantially along planes radially emanating from the axis of the tire rotation and a pneumatic tire built by the process.
2. Description of the Prior Art
Generally speaking, a radial tire comprises a carcass including one or more carcass plies whose cords are disposed substantially on tire meridian planes or radial planes emanating from the axis of rotation of the tire, a breaker mounted on the outer peripheral surface of the carcass along the equator of the tire, said breaker having cords disposed at an angle of 10.degree. to 30.degree. relative to the equatorial direction on plane of the tire, and a tread secured to the carcass so as to cover the breaker. The tire breaker acts to reinforce the tire crown.
Thus, the angle of the carcass cords of a radial tire relative to the equatorial direction is greatly different from that of its breaker cords. More particularly, the angular difference between the carcass cords and the breaker cords amounts to 60.degree. to 80.degree. in the case of radial tires, while the corresponding angular difference for conventional bias tires is about 10.degree.. As a result, the conventional onestep formation of bias tire green cases, in which all the tire making operations including the assembling and shaping of the various components into a green case are carried out on a cylindrical tire former, cannot be used for making the radial tire.
Accordingly, radial tire green cases have conventionally been made by a two-step process; namely, a first step of stretching and forming a carcass on a cylindrical former together with bead wires, chafers, and rubber stiffeners to be incorporated in the carcass followed by application of side rubber layers; and a second step of flexing the carcass formed in the first step into a toroidal shape having a cross section similar to that of a finished radial tire. In the second step, a breaker having a width substantially equal to the width of a tire tread and a tread rubber layer are successively secured to the outer crown portion of the carcass thus shaped. The two-step process must be used in making a radial tire green case because of the aforesaid large angular difference between the carcass cords and the breaker cords.
The tread rubber layer and the side rubber layer cooperate so as to form a continuous rubber layer covering almost the entire outer surface of the tire, including the crown portion which comes in contact with the road surface, the opposite shoulder portions, and the opposite side portions extending to the close proximity of the tire beads. The crown portion of the continuous rubber layer is required to have certain performance characteristics which are different from those required for the side portions. More particularly, high abrasion resistance is required for the crown portion because the crown portion contacts the road surface. On the other hand, high flexibility is required for the side portions because the side portions must flex upon loading. It should be noted that the extent of the flexing of the radial tire side portions upon loading is considerably larger than that of conventional bias tires.
Abrasion resistance and flexibility are incompatible properties of rubber materials so that different kinds of rubber materials are used for the crown portion and for the side portions of the radial tire. Usually, a rubber material having a Shore A hardness of 55.degree. to 75.degree. upon vulcanization is used for the crown portion, while a rubber material having a Shore A hardness of 40.degree. to 55.degree. upon vulcanization is used for the side portions. As pointed out in the foregoing, the side portions of the tread rubber layer are secured to the carcass in the first step, while the crown portion of the tread rubber layer is mounted in the second step. If a composite tread rubber layer having both the crown portion and the side portions in one piece is applied in the second step, the inner edge portions of the side rubber layers, which are comparatively closer to the axis of rotation of the tire, tend to wrinkle due to the difference in radius between the inner portion and outer portion of the tire. Such wrinkles making the tire shaping difficult.
Therefore, an object of the present invention is to obviate the aforesaid difficulties in the conventional process of making radial tires having a crown portion covered with one rubber material and side portions covered with a different rubber material by providing an improved process of efficiently making radial tires while ensuring high crack resistance and high separation resistance.